Chromium is commercially electroplated from electrolytes containing hexavalent chromium, but many attempts over the last fifty years have been made to develop a commercially acceptable process for electroplating chromium using electrolytes containing trivalent chromium salts. The incentive to use electrolytes containing trivalent chromium salts arises because hexavalent chromium presents serious health and environmental hazards--it is known to cause ulcers and is believed to cause cancer, and, in addition, has technical limitations including the cost of disposing of plating baths and rinse water.
The problems associated with electroplating chromium from solutions containing trivalent chromium ions are primarily concerned with reactions at both the anode and cathode. Other factors which are important for commercial processes are the material, equipment and operational costs.
In order to achieve a commercial process, the precipitation of chromium hydroxy species at the cathode surface must be minimized to the extent that there is a sufficient supply of dissolved, i.e., solution-free, chromium (III) complexes at the plating surface; and the reduction of chromium ions is promoted. U.S. Pat. No. 4,062,737 describes a trivalent chromium electroplating process in which the electrolyte comprises aquo chromium (III) thiocyanato complexes. The thiocyanate ligand stabilizes the chromium ions, inhibiting the formation of precipitated chromium (III) salts at the cathode surface during plating, and also promotes the reduction of chromium (III) ions. United Kingdom patent specification No. 1,591,051, described an electrolyte comprising chromium thiocyanato complexes in which the source of chromium was a cheap and readily available chromium (III) salt such as chromium sulphate.
Improvements in performance, i.e., efficiency or plating rate, plating range and termperature range, were achieved by the addition of a complexant which provided one of the ligands for the chromium thiocyanato complex. These complexants, described in U.S. Pat. No. 4,161,432, comprised amino acids such as glycine and aspartic acid, formates, acetates or hypophosphites. The improvement in performance depended on the complexant ligand used. The complexant ligand was effective at the cathode surface, to further inhibit the formation of precipitated chromium (III) species. In U.S. Pat. No. 4,161,432 it was noticed that the improvement in performance permitted a substantial reduction in the concentration of chromium ions in the electrolyte without ceasing to be a commercially viable process. In U.S. Pat. No. 4,278,512 practical electrolytes comprising chromium thiocyanato complexes were described which contained less than 30 mM chromium--the thiocyanate and complexant being reduced in proportion. The reduction in chromium concentration had two desirable effects, firstly, the treatment of rinse waters was greatly simplified and, secondly, the color of the chromium deposit was much lighter.
Oxidation of chromium and other constituents of the electrolyte at the anode are known to progressively and rapidly inhibit plating. Additionally, some electrolytes result in anodic evolution of toxic gases. An electroplating bath having an anolyte separated from a catholyte by a perfluorinated cation exchange membrane, described in United Kingdom patent specification No. 1,602,404, successfully overcomes these problems. Alternatively an additive, which undergoes oxidation at the anode in preference to chromium or other constituents, can be made to the electrolyte. A suitable additive is described in U.S. Pat. No. 4,256,548. The disadvantage of using an additive is the ongoing expense.
Japan published patent application 55-119192 describes an electrolyte for electroplating chromium which comprises trivalent chromium ions having a molar concentration greater than 0.01 M, one of aminoacetic acid, iminodiacetic acid, nitrilotriacetic acid and their salts, and one of dithionitic acid, sulphurous acid, bisulphurous acid, metabisulphurous acid and their salts. The electrolyte also contains alkali metal, alkali earth metal or ammonium salts for providing conductivity, and boric acid or borate for improving the plating and increasing the plating rate at high current densities.
U.S. Pat. No. 1,922,853 suggested the use of sulphites and bisulphites to avoid the anodic oxidation of chromium (III) ions. It was suggested that anodic oxidation could be prevented by using soluble chromium anodes and adding reducing agents such as sulphites, or by using insoluble anodes cut off from the plating electrolyte by a diaphragm.